1. Field of the Invention
The present invention is directed to magnet systems suitable for use in a nuclear magnetic resonance tomography apparatus, and in particular to a coil system for such an apparatus having an inner coil system and an outer coil system connected in series with opposite magnetic field directions, and a superconducting permanent current switch bridging the two coil systems.
2. Description of the Prior Art
A uniform fundamental or basic magnetic field is required for examination of a subject using the principles of nuclear magnetic resonance imaging. The nuclear spins of a subject disposed within the examination volume of the magnet are given a preferred direction by the fundamental magnetic field. Irradiation of the patient with an rf pulse, and the action of selected gradient magnetic fields, cause the nuclear spins to deviate from the preferred direction with a frequency and phase coding which permit a topical resolution of the examination subject to be made. After the action of the rf pulse and the gradient magnetic fields, the nuclear spins gradually re-orient to the preferred direction. The signals generated during this re-alignment are received by an antenna, and are supplied to an evaluation circuit which constructs a displayable image of the examination subject from these signals.
A superconducting coil system which surrounds the cylindrical examination volume can be used to generate the uniform magnetic field. Superconducting coils are of particular advantage when high fundamental field strengths are needed. The field strength in the examination volume of the superconducting coils usually is between 0.5 and 4 T.
Superconducting coils are frequently comprised of a plurality of individual coils, and can unintentionally convert into a normally conducting condition, commonly referred to as a quench. The rapid increase in impedance associated with a quench causes the energy stored in the coil system to be rapidly converted into heat in the coil, or in a region of the coil. Because the superconductor has a low heat capacity, the superconductor heats to an extremely pronounced degree, and thus experiences a further increase in impedance due to the heating. In addition to the heating, electrical over voltages can lead to the destruction of the insulation of the superconductor.
Many structures and methods are known in the art to prevent such unwanted quenching. Anti-quench measures are described, for example, in German OS No. 23 01 152, which uses ohmic protective resistances for this purpose. The use of semiconductor diodes for the same purpose is described in German OS Nos. 16 14 964 and 17 64 369. The semiconductor diodes are connected in parallel with the individual coils to limit the voltage.
These known anti-quench devices do not carry current in the superconducting operative condition. When a coil converts to the normally conducting condition, the anti-quench device accepts a portion of the current, and thus limits the voltage across the coil. The anti-quench device also accepts a part of the energy converted into heat. Because the energy stored in the coil system can be several million joules, it is desireable, in order to maintain the physical dimensions of the anti-quench device within a reasonable range, to convert the entire coil system to its normally conducting system under such circumstances, so that the energy stored in the coil system can be uniformly distributed and converted into heat over the entire coil system.
It is therefore known in the art to induce quench propagation when one region of a superconductor is quenched. Such a structure is described in European Application No. 0 115 797. In this system, the voltage of a network of anti-quench resistors is passively supplied to heating elements which are arranged in good thermally conducting contact with the coil system.
It is also possible to supply voltage from a power pack to the heating elements under the control of quench detectors. The heat generated by the heating elements cause the superconducting coil system to convert to its normally conducting condition, so that a uniformly distributed energy conversion is achieved.
A high external stray field is associated with the high field strengths in the inside of the coil system. To prevent unwanted interaction of this external field with magnetizable bodies and electronic systems in the area, the external field must be reduced to a low field strength. It is theoretically possible to shield the coil system from the surrounding area by ferromagnetic materials.
Another shielding approach is to provide a second coil system, which surrounds a first coil system, and which has a magnetic field direction opposite to that of the fundamental magnetic field. Such an actively shielded, superconducting coil system is described in U.S. Pat. No. 4,595,899 and in the article "Considerations in the Design of MRI Magnets With Reduced Stray Fields," Hawksworth et al, Applied Superconductivity Conference, Sept. 28-Oct. 3, 1986.
This type of active shielding significantly reduces the external magnetic field in the environment of the superconducting magnet. The area around the superconducting magnet having unsafe magnetic field levels for magnetizable bodies and electronic systems is thus also considerably reduced, producing a lower space requirement for the NMR system.
The inner coil system and the outer coil system may be separate circuits, or may form a common circuit. If the inner and outer coil systems are separate circuits, this compensates for chronologically varying external magnetic fields inside the respective coil systems due to a current change, because the magnetic flux .PHI. is constant in a closed superconducting circuit because the electric field E cannot have any component along the superconductor. This is expressed in the equation ##EQU1## wherein A is the cross-sectional area of the circuit, and U is the circumference of the circuit.
A change in the magnetic induction B of the field strength is therefore compensated by a current change in the superconductor. Essentially uniform, external magnetic field changes which, for example, are caused by moving magnetizable bodies such as automobiles and street cars thus do not cause any change in the inner, uniform fundamental magnetic field, assuming that the inner coil system is designed such that it alone generates the uniform magnetic field. A disadvantage of the use of such separate circuits for the two coil systems is that the external field may be undesireably increased if only one of the two coil systems converts to the normally conducting condition.
For safety reasons, both coil systems are therefore connected in series in actively shielded magnets of this type. Upon the quench of one of the coil systems, the current in the entire circuit, and thus the external field, are reduced. The disadvantage of this interconnection of the systems is that the inner fundamental magnetic field is no longer insulated from external magnetic field disturbances. Upon the occurrence of magnetic field changes which effect a current change, only the magnetic flux in the annular gap between the coil systems stays constant because, due to the series connection of the coil systems, only this annular gap is surrounded by a closed, superconducting circuit. The necessary constancy of uniformity of the fundamental magnetic field is thus no longer insured.